Not applicable.
The invention relates to a method and an apparatus for mixing dilution liquid into a stock flow in a paper or board machine.
With respect to the prior art, we refer to the publications DE 19723861 and FI 901593.
It has become clear that with the development of measuring devices on the market ever higher requirements are set for the accuracy of control of the basis weight profile. Today, the dilution spacing in a so-called dilution headbox is about 32-75 mm, and it is not possible to reduce it any more if fibre-containing white water is used as dilution water, because dilution feed ducts which remain open by means of white water cannot be accommodated between tube rows with a dense spacing.
As a solution it is proposed that, when needed, dilution is changed to comprise two stages such that coarse control is carried out by means of white water and fine control is carried out by means of raw water.
The increasing requirement of control accuracy calls for an increasingly denser dilution spacing and, therefore, still narrower dilution feed ducts. If white water is used as dilution water, narrow dilution ducts clog easily. Clogging problems are not encountered with raw water, but its xe2x80x9cfull-scale usexe2x80x9d is not economical and sensible for environmental reasons.
The idea of the two-stage dilution is to correct large basis weight profile errors by a large amount of white water and small profile errors by a small amount of raw water. A good raw water economy is achieved by this means in a paper mill. Another benefit of the two-stage arrangement is the good possibility of adjusting the basis weight profile. The entire valve control area can be made use of and control valves of an optimum size can be selected for both control operations. Coarse control is carried out in a tube bank after an inlet header, as in the conventional headbox. In the first dilution stage, the control spacing can be increased, for example, to 120 mm such that one dilution member feeds two tube rows. Course control corrects major errors in the shape of the profile, such as, for example, profile errors arising from web shrinkage. The small errors which remain in the profile after coarse control are rectified by means of fine control dilution in the second stage.
Fine adjustment is carried out as turbulence generator dilution by supplying some or each of the tubes of the turbulence generator with dilution liquid. A very small amount of dilution liquid is needed for rectifying the remaining small errors, so raw water or clarified white water obtained from a fibre recovery unit can be used economically as dilution water in fine control. Since, for example, raw water does not contain contaminating or clogging particles, the dilution ducts can be provided in very narrow spaces. Moreover, the control valves and the actuators operating the valves can be ordinary standard devices available on the market, which devices are considerably less expensive than conventional dilution valves and actuators.
Minimum local dilution with raw water can be almost 0% and maximum local dilution need not be high because the consistency of raw water is 0% and the remaining error to be corrected is small. Thus, the amount of the more expensive raw water consumed is very small. No separate circulation is required for the feed of raw water.
The price of the arrangement disclosed hardly differs at all from the price of the conventional dilution headbox. The proposed arrangement uses half the number of expensive dilution valves and actuators.
Thus, mixing units are prior known in which dilution water and stock passed from the inlet header of the headbox are mixed and the combined flow is passed further onwards in the headbox and onto a forming wire. Points of supply of dilution liquid are situated in different positions of width across the headbox and, thus, depending on the density of the dilution points placed across the width of the headbox, desired resolution is obtained for control of the basis weight of the web.
Thus, this application proposes using dilution in at least two stages. Coarse control of the basis weight profile is carried out in the first stage of dilution and fine control is carried out in the second stage of dilution. White water is used as dilution water in the first stage and the valves are arranged with a less dense spacing in the first stage than in the second control stage in which the valves are arranged with a denser spacing than in the first dilution stage. An advantage of the arrangement is that the valves of the second stage can have a construction that demands less precision and thus be less expensive than the valves of the first stage. They do not clog because fibre-free dilution water is used in the second stage. The valves can thus contain smaller ducts. They do not demand much space.
Within the scope of the invention, it is also possible to use control with three or more stages, but the most advantageous control arrangement is two-stage adjustment of the dilution liquid.
The headbox structure of the paper or board machine can advantageously be as follows:
a) stock is passed into a stock inlet header which tapers towards its outlet end in a conventional manner,
b) the stock flow is passed from the stock inlet header into a tube bank and further through the tube bank into an intermediate chamber,
c) the stock flow is passed from the intermediate chamber further into a turbulence generator and from the turbulence generator further through a slice cone onto a forming wire.
In the following, the invention will be described with reference to some advantageous embodiments of the invention shown in the figures of the accompanying drawings, to which the invention is, however, not intended to be exclusively confined.